1. Field of the Invention
The present invention relates to a three-piece oil-ring consisting of a pair of side rails and an expander provided between the paired side rails.
2. Description of the Prior Art
Three-piece oil-rings have been known including a pair of side rails separated by an expander extending in a circumferential direction with waves in a radial direction. Both ends of the expander are opposed to each other. Side rail engaging portions of the expander are formed on both sides, axially, of radially inner portions of the expander so as to protrude in an axial direction and to extend in a circumferential direction. The side rail engaging portions are engaged with inside surfaces of the pair of side rails, thereby transmitting the expanding spring force of the expander to the paired side rails, pushing the paired side rails slildably against a cylinder bore surface.
In the composite oil-ring including the above-mentioned radially waving expander, since the expander itself has rigidity, the side rail engaging portions formed on opposite sides of flat portions at the radially inner portions of the expander cause each other to deform. Also, typical cylinder bores do not have perfectly straight axial walls. Therefore, one of the paired side rails may contact the cylinder bore surface while the other one does not. The result is an increase in oil consumption.
Since this problem arises due to the fact that the side rail engaging portion at one side of the expander is deformed by the opposite side rail engaging portion, various inventions to reduce rigidity of the expander have been proposed. For example, Japanese Utility Model Publication SHO 57-38956 discloses a composite oil-ring wherein spring arms extend from both upper and lower portions of an expander and tip portions of the arms contact an inside surface of a rail ring and expand while a center portion of the rail ring is supported by a waving portion of the expander.
In the above composite oil-ring, since a rail engaging portion has very low rigidity, the rail engaging portion at one side of the expander receives little deforming influence from the opposite rail engaging portion. However, since the rigidity of the rail engaging portion is very low, fluttering due to a resonant phenomenon may happen at high engine speeds, causing an increase in oil consumption. As is apparent from the above description, the rigidity of the side rail engaging portion is required to be set not too high and not too low to minimize oil consumption.